Fluorescent lamps are used in a number of applications including, without limitation, backlighting of display screens, televisions and monitors. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (typically on the order of 700 to 1,600 volts) for a short period of time to ionize a gas contained within the lamp tubes and fire or ignite the lamp. This starting voltage may be referred to as a strike voltage or striking voltage. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on.
In liquid crystal display (LCD) applications, a backlight is needed to illuminate the screen so as to make a visible display. Backlight systems in LCDs or other applications typically include one or more CCFLs and an inverter system to provide both DC to AC power conversion and control of the lamp brightness. Even brightness across the panel and clean operation of inverters with low switching stresses, low EMI, and low switching losses is desirable. While CCFL backlighting is common, other fluorescent lamps such as external electrode fluorescent lamps (EEFLs) or flat fluorescent lamp (FFLs) may be utilized in place of CCFLs, with somewhat similar requirements. With the increasing size of LCDs and the high screen brightness requirements for better display quality, the power consumption of the backlight system becomes a major factor in the total system power consumption of an LCD based monitor or television.
While the above has been described in relation to a CCFL or other fluorescent lamp, various LCD based monitors are also produced utilizing Light Emitting Diodes (LED) s to provide a backlight system. These LED based systems may be constituted of white LEDs or a plurality of colored LEDs controlled to provide an apparent white light. The term luminaire is used herein to describe a light source, irrespective of whether it is fluorescent light based or LED based, without limitation.
In prior art systems, the incoming power line voltage is first rectified, and a power factor corrector (PFC) is typically provided. The rectified voltage is then converted to a low voltage, typically on the order of 24 volts, and the low voltage is fed to a backlight controller. The backlight controller controls a switching network connected to the primary side of a transformer, and the luminaires are connected to the secondary side of the transformer. The backlight controller is operative to produce the necessary AC driving voltage by controlling the operation of the individual switches of the switching network. Such an operation is described, for example, in U.S. Pat. No. 5,615,093 issued Mar. 27, 1997 to Nalbant, the entire contents of which is incorporated herein by reference.
Unfortunately, the above architecture leads to excessive power loss, since an incoming AC line voltage is first converted to a high voltage DC, the high voltage DC is then converted to a low voltage DC, and the low voltage DC is then again converted to a higher AC voltage for driving the luminaires. In a move to reduce power consumption, an architecture called LCD Integrated Power Systems (LIPS) has been developed. For example, ON Semiconductor has published a GreenPoint reference design, certain selected portions of which are shown in FIG. 1. In particular, the LIPS architecture of FIG. 1 comprises: An A/C line source 10; an EMI filter 20; a full wave rectifier 30; a PFC circuit 40; a switching network 50; a transformer 60; a backlight controller 70; current sensing and over-voltage detecting circuitry 80; a balancing network 90; a plurality of luminaires 100, each illustrated without limitation as a CCFL; and a plurality of isolation circuits 110. PFC circuit 40 comprises a transformer, a PFC controller, a resistor, an electronically controlled switch, a diode and an output capacitor. Switching network 50 comprises a plurality of electronically controlled switches, illustrated, without limitation, as NMOSFETs. Transformer 60 exhibits a single primary winding magnetically coupled to a pair of secondary windings. Current sensing and over-voltage detecting circuitry 80 comprises a pair of capacitor voltage dividers connected to a secondary side common point, and a resistor connected between the two secondary windings and the secondary side common point. Balancing network 90 comprises a plurality of balancing transformers, each associated with a particular luminaire 100. Balancing network 90 is arranged so that current is received at one end of each luminaire 100 via a respective balancing transformer primary winding, and the secondary windings of the balancing transformers are connected to form an in-phase closed loop. The arrangement of balancing network 90 is further taught in U.S. Pat. No. 7,242,147 issued Jul. 10, 2007 to Jin, the entire contents of which is incorporated herein by reference. In an exemplary embodiment, backlight controller 70 is constituted of an LX 6503 Backlight Controller available from Microsemi Corporation, Garden Grove, Calif. The second end of each luminaire 100 is connected to the secondary side common point.
The output of A/C line source 10 is received by EMI filter 20, and the output of EMI filter is connected to the input of full wave rectifier 30. The output of full wave rectifier 30 is fed to PFC circuit 40, and the output of PFC circuit 40 is fed to switching network 50. The output of switching network 50 is connected to the primary winding of transformer 60, and the secondary windings of transformer 60 are connected to each of the plurality of CCFL lamps constituting luminaires 100 via balancing network 90. The current sense output of current sensing and over-voltage detecting circuitry 80 is connected to a respective input of backlight controller 70, and the over-voltage detecting output of current sensing and over-voltage detecting circuitry 80 is connected to a respective input of backlight controller 70. A PWM dimming input, denoted PWM DIM, an analog dimming input, denoted ANALOG DIM, an enable input, denoted ENABLE, and a synchronization input, denoted SYNCH, preferably sourced by a separate video processor (not shown), are further fed to respective inputs of backlight controller 70. The in-phase closed loop formed by the secondary windings of the balancing transformers of balancing network 90 is also coupled to a respective input of backlight controller 70. Backlight controller 70 exhibits a plurality of outputs, which are each fed via a respective isolation circuit 110 to the control input of the respective electronically controlled switch of switching network 50.
Switching network 50 is preferably a full bridge network comprising 4 electronically controlled switches. The full bridge network can be replaced with a half bridge switching work, thereby reducing cost, however there is often a penalty of severe ringing at turn off due to the hard switching behavior with resulting high switching losses and strong EMI emissions. These problems can be mitigated with additional circuitry; however this again increases the cost. Alternatively, a resonant half bridge switching method may be implemented; however resonant operation varies the switching frequency with operating conditions which is not favored in many display applications.
The output of PFC circuit 40 is normally in the range of 375V to 400 VDC, and in the LIPS architecture of FIG. 1, this voltage is directly used to drive the primary winding of transformer 60 responsive to switching network 50, without requiring a voltage step down. This approach thus provides significant cost savings and efficiency improvements as opposed to earlier prior art applications because of the removal of the DC to DC converter stage for the inverter input.
One of the challenges of the LIPS architecture of FIG. 1 is the safety isolation requirement. In particular, PFC circuit 40 is on the mains input side whereas backlight controller 70 and the respective input signals for backlight controller 70 are on the secondary side of transformer 60, and an electrical insulation of >3750 VRMS between the two sides is mandatory for consumers' safety. Advantageously, backlight controller 70 thus does not require isolation for the feedback signals from current sensing and over-voltage detecting circuitry 80, nor is isolation required for any of the PWM dimming input, analog dimming input, enable input and synchronization input signals received from the video processor. Transformer 60 provides the required isolation between PFC circuit 40 and the circuitry associated with backlight controller 70.
The LIPS architecture of FIG. 1 however exhibits costly isolation circuitry between backlight controller 70 and switching network 50, since the signals driving each of the electronically controlled switches of switching network 50 must be driven with relatively sharp edges, and with sufficient drive current to ensure rapid opening and closing of the electronically controlled switch. Isolation circuits 110 are thus typically implemented with isolation transformers or high speed photo couplers, which are quite costly as compared with low speed opto-isolators which, if used in such an implementation, would produce significant distortions to the rising and falling edges of the high speed signals.